Engine emission control systems may include one or more exhaust catalysts to address the various exhaust components. These may include, for example, three-way catalysts, NOx storage catalysts, light-off catalysts, SCR catalysts, etc. Engine exhaust catalysts may require periodic regeneration to restore catalytic activity and reduce catalyst oxidation. For example, catalysts may be regenerated by injecting sufficient fuel to produce a rich environment and reduce the amount of oxygen stored at the catalyst. As such, fuel consumed during catalyst regeneration can degrade engine fuel economy. Accordingly, various catalyst regeneration strategies have been developed.
One example approach is shown by Georigk et al. in U.S. Pat. No. 6,969,492. Therein, an emission control device includes catalytic converter stages generated by at least two catalysts arranged in series. Specifically, the catalytic stages include a three-way catalyst arranged in series with (e.g., upstream of) a NOx reduction catalyst. The different ammonia storage performance of the different catalysts enables NOx reduction to be improved and reduces the need for catalyst regeneration. Another example approach is shown by Eckhoff et al. in WO 2009/080152. Therein, an engine exhaust system includes multiple NOx storage catalysts with an intermediate SCR catalyst, and an exhaust air-to-fuel ratio is continually alternated between rich and lean phases based on differences between an air-to-fuel ratio upstream of a first NOx storage catalyst and an air-to-fuel ratio downstream of a second NOx storage catalyst.
However, the inventors herein have identified potential issues with such approaches. For example, the inventors have recognized that the regeneration control may degrade during the idle-stop operations performed during a vehicle drive cycle. In particular, during an idle-stop when the engine is deactivated and fuel is shut off for the shut-down, the engine still spins a few more times. This spinning pumps air over an exhaust three-way catalyst, causing the catalyst to become oxidized and degrading its ability to reduce NOx when the engine is reactivated. Likewise, before the engine is restarted from idle-stop, the engine is spun a few times, providing another opportunity during which air can be pumped over the exhaust catalyst. And while enrichment can be used to quickly regenerate the three-way catalyst upon engine reactivation, the enrichment leads to a fuel penalty. In addition, delays in engine restart can degrade engine performance.
In one example, some of the above issues may be at least partly addressed by a method for reducing exhaust catalyst oxidation during a cylinder deactivation, thereby reducing an amount of regeneration required upon reactivating the engine cylinders. Specifically, the method may include selectively deactivating one or more engine cylinders via deactivatable fuel injectors. Then, during cylinder deactivation, water may be injected at the one or more deactivated engine cylinders to reduce oxidation of a first exhaust catalyst. In one example, the first exhaust catalyst may be a three-way catalyst. An engine controller may determine an injection timing and injection amount for the water injection during the cylinder deactivation. Upon engine cylinder reactivation (e.g., after an idle-stop), water injection may be stopped and the one or more deactivated engine cylinders may be reactivated with a combustion air-to-fuel ratio based on an estimated ammonia content of a second exhaust catalyst. In one example, the second exhaust catalyst may be an SCR catalyst. For example, the combustion air-to-fuel ratio may be less rich as the ammonia content of the second exhaust catalyst increases. In this way, by injecting water and reducing catalyst oxidation during a cylinder deactivation event, fuel penalty from enrichment during cylinder reactivation may be reduced while maintain a required NOx emission level.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.